Cancer Antigen (CA) 125, Serum. CA-125 is not fully recognized as a screening test for ovarian cancer because it can be negative in ovarian cancer and may be positive in conditions other than ovarian cancer. However, CA-125 testing has been shown to help detect certain ovarian cancers at an earlier stage than had testing not been performed. The rate of change in your CA-125 levels can indirectly measure the level of cancer